Add optional header

src/openrct2/core/Optional.hpp

@@ -0,0 +1,29 @@
+/*****************************************************************************
+ * Copyright (c) 2014-2019 OpenRCT2 developers
+ *
+ * For a complete list of all authors, please refer to contributors.md
+ * Interested in contributing? Visit https://github.com/OpenRCT2/OpenRCT2
+ *
+ * OpenRCT2 is licensed under the GNU General Public License version 3.
+ *****************************************************************************/
+
+#pragma once
+
+#ifdef __has_include
+#    if __has_include(<optional>)
+#        include <optional>
+#    elif __has_include(<experimental/optional>)
+#        include <experimental/optional>
+#    else
+#        error Missing <optional>
+#    endif
+#else
+#    error __has_include is not defined
+#endif
+
+// `optional` and friends will be available in NS `::opt`
+#if __has_include(<optional>)
+namespace opt = std;
+#elif __has_include(<experimental/optional>)
+namespace opt = std::experimental;
+#endif